Conventionally known water treatment systems generate an electric discharge in water to produce bactericidal factors. For example, Patent Document 1 discloses this type of water treatment system. The water treatment system of Patent Document 1 purifies water for humidification stored in a tank. Specifically, this water treatment system generates an electric discharge in the water stored in the tank, and sterilizes the water in the tank by using bactericidal factors, such as hydroxyl radicals, produced by the electric discharge.